1. Field of the Invention
The present invention relates to an optical disc cutting apparatus and a method for manufacturing an optical disc.
2. Description of the Related Art
Recently, optical discs increasingly have been applied to an AV (audio visual) field. For example, in DVD (Digital Versatile Disc) mainly for movie contents, write-once type and re-writable type formats such as DVD-R, DVD-RAM, DVD-RW, have been developed and are becoming widespread as a next generation recording apparatus of VTR.
In the future, with the availability of BS digital broadcasts or broadband communication, large capacity optical disc formats capable of recording higher quality compressed pictures and optical disc formats having a small and portable size even with the same capacity and exhibiting high affinity to the network are expected.
In order to realize a high-density optical disc as mentioned above, it is indispensable to develop materials and formats capable of recording with high density as well as to develop an optical disc substrate suitable for high definition, high precision and high density. A cutting method that is a headstream step thereof plays a key role in achieving high density. More specifically, it is important to produce narrow-pitch tracks and fine prepits on an original master precisely.
It is important to minimize an optical spot and to align the position of optical spots with high precision. The former can be realized by shortening a wavelength of a laser beam source and making the NA of an objective lens to be high. The latter can be realized by making a feeding mechanism of an objective lens to have high precision, lowering the vibration of a cutting apparatus and enhancing the precision of the deflection in the case where an optical disc needs wobble, and the like.
In the explanation mentioned below, an optical disc cutting method will be explained as an embodiment of an optical disc cutting apparatus for specifically realizing the method. FIG. 9 shows an example of a conventional optical disc cutting apparatus and shows only basic component elements.
Reference number 1 denotes a laser beam source from which a predetermined light intensity of parallel light beam Lc is emitted. In general, Ar gas laser is used. A light source of the visible light region, in which the wavelength to be used is in the 500 nm range, conventionally has been used. However, a shorter wavelength has been used in accordance with the needs of high density, and recently, light sources of ultraviolet spectrum of wavelength between 200 nm and 300 nm are used.
Reference number 2 denotes an EO modulator (EOM: Electro-Optic Modulator) capable of changing a light intensity in an analog way by an applied voltage. In general, by applying two levels of voltage, binary level of light intensity corresponding to ON and OFF is selected to form prepits of the optical disc by modulating in a digital way. And grooves are formed by keeping ON level. Reference number 3 denotes an EO deflector (EOD: Electro-Optic Deflector) for deflecting the angle in which the light beam progresses by the applied voltage and displacing an optical spot for cutting formed on an original master 100 by an objective lens 5 mentioned below. The original master is hereinafter referred to as “optical disc” or “optical disc original master.”
As materials for the EO modulator 2 and the EO deflector 3, crystalline materials having a so-called Pockels effect are used in which refractive index anisotropy is changed when high voltage is applied. In order to obtain a deflection amount or modulation amount that is effective in cutting, it is necessary to apply high voltage of, for example, ±200V to a portion between two electrodes provided on the EO element. Furthermore, instead of the EO element, an acoustic-optical element system called an AO (Acoustic-Optic) element may be used.
Light beam Lc passing through the EO modulator 2 and the EO deflector 3 are reflected by a mirror 4 and converged by the objective lens 5 having a high NA, thus forming an optical spot Sc on an original master 100 whose rotation is controlled by a motor 6. The mirror 4 and the objective lens 5 are formed of an optical head for cutting held by a mechanism (not shown in the drawing) and transported continuously and precisely corresponding to the track pitch in accordance with the rotation of the motor 6 by a transport system (not shown in the drawing). In cutting, a CAV (constant angular velocity) mode in which the rotation speed is constant or a CLV (constant linear velocity) mode in which the linear velocity is constant is used in general.
By continuously transporting one track pitch per rotation of the original master precisely, a spiral track is formed on the original master. In the case of the CAV mode, the light intensity of the laser beam source 1 is controlled to be an appropriate value according to a cutting radius. The optical disc formed in the transport direction as shown in FIG. 9 is subjected to cutting by the CAV mode with the lead-in located at an inner position.
Reference number 101 denotes a formatter of an optical disc that forms prepits or grooves on the original master by controlling the EO modulator 2 and the EO deflector 3 while controlling the light intensity of the laser beam source 1 and the rotation speed of the motor 6, thereby forming a desired optical disc format. An output signal Vem of the formatter 101 is transmitted to an EOM driver 102. The EOM driver 102 directly amplifies Vem or switches on a high voltage power source to generate an EOM drive signal Vemd and feed it to the EO modulator 2, thus modulating the light intensity of the light beam Lc. Herein, the light beam Lc is made ON/OFF by “H”/“L” control of the signal Vem.
Furthermore, the deflection signal Ved of the formatter 101 displaces the position of the optical spot on the original master in the radial direction. That is, in accordance with polarities, “positive, zero, negative” of the deflection signal Ved, “inner direction displacement, neutral, outer direction displacement” are controlled, respectively. The EOD driver 103 receives a deflection signal Ved and directly amplifies thereof (hereinafter, referred to as “analog deflection mode”) or switches on a high voltage power source (hereinafter, referred to as “digital deflection mode”) to generate the EOD drive signal Vedd and feed it to the EO deflector 3, thus controlling the deflection amount of the light beam Lc.
Practically, when clock and address information are superimposed on the grooves by wobbling, small displacement of several tens nm order is required for the wobbling on the original master of the optical disc. Since the applied voltage is as small as several tens V, the analog deflection mode using an amplifier is employed. In order to achieve a step-wise displacement amount of several hundreds nm like a sample servo method, etc., the digital deflection mode is employed. In the latter case, in order to simplify a circuit, the deflection signal Ved is formed of two control signals showing the deflection to the inner position or deflection to the outer position, and according to the signal, the voltages are applied to the two terminals of the EO deflector 3 alternatively in a way by exchanging the conduct to the positive pole or the negative pole of the power source of, for example, about +200V, thereby obtaining an EOD drive signal of ±200V, equivalently.
Light beam Li shown by broken lines in FIG. 9 show the state in which the EO deflector 3 deflects the light beam Lc when the deflection signal Ved is positive. As a result, the light spot Sc is displaced from the position Sc to the inner position Si.
FIG. 10A and 10B show two specific examples of the optical disc that can be formed by the above-mentioned optical disc cutting apparatus. An operation of one of the examples of the optical discs shown in FIG. 10A will be explained with reference to FIGS. 11A and 11B.
FIG. 10A shows an example of an optical disc substrate of a sample servo method with grooves, and FIG. 10B shows an example of an optical disc substrate of a land-groove continuous servo method. In both figures, the upper side is an inner position and the lower side is an outer direction and recording/reproducing of optical spots scans from left to right.
An optical disc substrate shown in FIG. 10A is a substrate suitable for a DWDD (Domain Wall Displacement Detection) technique that is one of a super-resolution techniques of a magneto-optical disc. In the case of DWD optical disc, it is necessary to weaken magnetic coupling (reducing the magnetic anisotropy) between adjacent recording tracks. Therefore, when the DWDD optical disc is manufactured, it is necessary to carry out the initialization for weakening the magnetic coupling between the adjacent recording tracks (hereinafter, referred to as “annealing”) before recording information signals. Grooves are used at the time of the annealing.
The component unit of this optical disc is a segment as shown in the drawing. The segment includes a servo region and a data region. One track includes 1000 segments or more. In the servo region, first wobble pit 11, second wobble pit 12 and address pit 13 for sample servo are disposed. The tracking of the sample servo method is controlled so that the recording/reproducing light spot scans the middle portion between the first wobble pit 11 and the second wobble pit 12.
The address pit 13 is dispersed in a plurality of segments in which the presence or absence of the pits in every segment is collected and subjected to error correction and then reproduced as a track address. The information track of the data region includes groove 14. A portion between the information tracks is a land 15. Since the track pitch in this case is as small as 0.6 μm or less, it is difficult to form wobble pits independently in each track. Therefore, the wobble pits are shared by adjacent tracks. In other words, the track is basically classified into two kinds, that is, Ta and Tb as shown in the drawing.
When the track Ta is assumed to be an even number track 2m, the recording/reproducing light spot 16 scans so that the first wobble pit 11 is located left and the second wobble pit 12 is located right with respect to the recording/reproducing light spot 16. The track Tb is an odd number track 2m+1, and contrary to track Ta, the recording/reproducing light spot scans so that the first wobble pit 11 is located right and the second wobble pit 12 is located left with respect to the recording/reproducing light spot 16. The track address is devised to be disposed in the segment so that the track Ta and the track Tb can be reproduced independently (detailed explanation is omitted herein because it is not within the scope of the present invention).
The above-mentioned annealing weakens the magnetic coupling by scanning the land portion 15 at high power by using a light spot for annealing having a spot diameter smaller than that of the recording/reproducing light spot 16. The reason why the size of the light spot for annealing is reduced is to narrow the annealing width even if the track pitch is narrow so as to broaden the track width of DWDD operation area remaining in the groove 14 and to secure the recording/reproducing performance. Therefore, for a light source for annealing light spot, a laser having a wavelength of 405 nm and an objective lens having a NA of 0.75 to 0.85 are used, thereby reducing the diameter of the light spot to the diameter that is about half as compared with the recording/reproducing optical spot having a wavelength of 650 nm and NA of 0.6.
The tracking at the time of annealing is carried out by the continuous servo method using a push-pull signal from the land 15. Even if recording/reproducing wobble pits are used, tracking can be carried out with respect only to grooves but cannot be carried out with respect to lands. If wobble pits only for annealing are disposed, the redundancy of the servo region increases, and eventually high density cannot be attained. Therefore, it is preferable that the tracking is carried out on the land 15 as mentioned above. As the land 15 is continuous, so the tracking precision is increased. Further, it is effective in separating the grooves geometrically and thermally. Therefore, even in a sample servo method, the groove 14 is very effective in an optical disc.
The optical disc substrate shown in FIG. 10B is an example of the optical disc substrate of a land-groove continuous servo method and this is an example corresponding to a DVD-RAM format that is a re-writable type phase-change optical disc. The component unit of this optical disc is a sector and a plurality of sectors constitute a track. The sector includes an address region and a data region. First, the data region is formed of a track including groove 23 or land 24 and has a spiral configuration in which a groove track Tg (2n: even number track) including groove 23 and a land track Tl (2n+1:odd number track) including land 24 are scanned alternately.
Furthermore, in the address region, addresses called CAPA (Complementary Allocated Pit Address) are offset by half of the track pitch. Reference number 21 denotes a LG common address portion shared by the land track Tl that is located at the inner direction seen from a certain groove track Tg. The reference number 22 denotes a GL common address portion shared by the land track Tl that is located at the outer position seen from a certain groove track Tg. Each address portion is formed of prepit groups showing address information. Although not shown in the drawing, onto the groove 23, a clock by the wobble with the amplitude smaller by one digit than the track pitch is superimposed. In order to realize this, by inserting the deflector that is different from the EO deflector 3 into the light beam Lc shown in FIG. 9, the light beam Lc is deflected by the above-mentioned analog deflection mode using the clock superimposing signal separately output from the formatter 101.
FIGS. 11A and 11B are views showing an operation of cutting an optical disc shown in FIG. 10A. When the cutting of this optical disc is carried out, for deflecting the EO deflector 3, the digital deflection mode as mentioned above can be employed (EOD driver 103 is a switching type). FIG. 11A is a timing chart showing a control signal of the cutting apparatus. In the timing chart, from the upper part, (a1) inner position deflection signal Vedi, (a2) outer position deflection signal Vedo, (a3) EOD drive signal Vedd, and (a4) EOM drive signal Vemd are shown.
FIG. 11B shows an optical disc original master to be formed and the path of an optical spot for cutting. In FIG. 11B, from the upper part, (b1) track Ta and (b2) track Tb have that have already been formed, and (b3) Ta track are shown. Since the deflection is carried out in the Ta track like a path shown in FIG. 11B, in the track Tb, the first wobble pit 11 and the second wobble pit 12 are not formed and only the address pit 13 are formed as necessary. (b3) denotes a track Ta that has just been formed in an optical spot 31 for cutting in which prepits and grooves are shown in a hatched pattern.
In the formation of the track Ta shown in (b3), from the time t6 to the next t1 for forming groove 14, the EO modulator 2 emits a light spot 31 for cutting (increase the light intensity) and the EO deflector 3 carries out cutting continuously without deflection. The servo region is formed from the time t1 to t6. The EOM drive signal Vemd emits light when the first wobble pit 11, second wobble pit 12 and address pit 13 are formed in the section.
Furthermore, the EOD drive signal Vedd makes the inner position deflection signal Vedi to be “H” during the time t2 to t3, so that the deflection voltage is output in the direction in which the first wobble pit 11 are displaced toward the inner position; makes the outer deflection signal Vedo to be “H” during the time t4 to t5, so that the deflection voltage is output in the direction in which the second wobble pit 12 are displaced to the outer position; and during the rest of the time, the deflection voltage is output to be 0 so that the displacement becomes 0. As a result, the optical spot 31 for cutting scans the original master 100 passing a path shown by an arrow, forming the prepits and grooves shown in a hatched pattern in combination with the light emitting timing. Note here that the ON/OFF timing of the actual EO drive signal Vemd is adjusted so as to be disposed precisely in the position shown in FIG. 11B on the original master 100.
However, the above-mentioned optical disc cutting apparatus has the following problems. That is:                (Problem 1) When the EO deflector is changed stepwise in the digital deflection mode (hereinafter, referred to as “a step deflection”), unnecessary deflection (fluctuations), which seems to be the relaxation oscillations of the EO crystalline material, is generated. First of all, as shown in FIGS. 10A and 10B, this phenomenon was found through the measurement of tracking error signal of the optical disc, the observation by an electron microscope, or the like. Next, observation was carried out by preparing a deflection amount detector capable of observing the deflection amount in the optical path of the optical disc cutting apparatus. As a result, it was found that the unnecessary deflection is a fixed pattern of noise in the range from, for example, 500 kHz to 700 kHz and that the signal disappears by attenuation about 10 μs after the step deflection. The unnecessary deflection occurs in the header portion of the segments shown by the duration of t7 in FIG. 11B.        
The deflection amount is relatively large, for example, 10 to 20 nmpp. Assuming that grooves having a relatively large width (width of the bottom) of 350 nm and a groove depth of 30 nm is formed at the track pitch of 0.50 μm (500 nm), when the side wall of the groove has a tilt angle of 30 degrees, the occupied width of the tilting portion is 30 nm×√{square root over (3)}×2=104 nm. As a result, the width of the upper side of the land formed between the grooves becomes at most 46 nm (=500−350−104). So the deflection amount 20 nmpp is not a negligible value. Unless the width of the upper side of the land is maintained to some extent, the above-mentioned annealing cannot be carried out normally, thus causing a severe problem.
As to the cutting, in addition to this, the variations in transporting of the optical head for cutting, in the cutting power, in the sensitivity of the photoresist and in development may cause a variation of the width of the land, thus radically reducing the margin assigned thereto. Needless to say, it is also important to secure the width of the land in the following molding process. Therefore, it is necessary to suppress the variation of the width of the land as much as possible. So this unnecessary deflection observed herein should be suppressed at any cost.
Besides, if the unnecessary deflection is actually present, the grooves, which should essentially be formed straight, are fluctuated. Therefore the disturbance of the signal detection (for example, envelope variation) or the disturbance of the tracking error signal (TE) occurs, thus deteriorating the signal processing or the servo performance. Furthermore, the groove fluctuation itself may change the stress to a recording film, or may affect the thermal distribution at the time of recording/reproducing, to deteriorate the off-track margin at the time of recording and reproducing. In particular, since the header position of the segment and sector is an important part where signals for synchronization are placed, it is necessary to keep recording and reproducing reliability in this portion to be high. Also in this sense, it is indispensable to suppress the unnecessary deflection.                (Problem 2) Furthermore, when the observation is carried out by using the deflection amount detector, two kinds of unnecessary deflections other than that explained in the problem 1 were observed. One is an unnecessary deflection component of rather slower fluctuation having a frequency of 1 Hz or less, which seems to be caused by the fluctuation of air existing in the path of light beam in the cutting apparatus; and another is a deflection component of the fluctuation having frequency of about 100 Hz to 1 kHz, which seems to be caused by the mechanical vibration of each optical part of the cutting apparatus. The former generates a large displacement of about 100 nm or more on the original master. But by covering the cutting apparatus as a whole and suppressing the air flow, the fluctuation can be reduced to about 1/10. However, heat etc. of the laser beam source in the cutting apparatus remains and the airflow cannot absolutely be stopped.        
Furthermore, as a measure to this problem, the cutting apparatus may be miniaturized and an optical path itself may be shortened. However, this measure cannot suppress the entire fluctuation completely because of the physical dimension limitation of each optical element constituting the cutting apparatus. This fluctuation of 1 Hz or less does not directly cause the variation in track pitches with respect to the rotation speed of the original master, but may cause fluctuation in the absolute position of the track. The latter fluctuation of 100 Hz to 1 kHz is in the level explained in the problem 1. Since this is in the range of generating the fluctuation in one rotation of the original master, the variation of the track pitches may be caused.
These two kinds of fluctuations are observed similarly in the tangential direction besides in the radial direction of the original master. The fluctuation in the tangential direction causes jitter at the header position and the end position of the prepits or grooves. The fluctuation as mentioned above needs to be suppressed sufficiently in order to obtain a highly precise original master of the optical disc.                (Problem 3) In the above-mentioned problem 1 and problem 2, light beams used for cutting are affected by unnecessary deflection. In addition, it was found that the unnecessary optical variation (variation of light intensity) that is synchronous to the deflection of the problem 1 and problem 2 is observed. The laser beam source itself is controlled to an appropriate level by optical power servo, however, the unnecessary deflection may be caused by the following fluctuation of the EO modulator, the EO deflector, the optical components property or an optical path. Since the variation of the light intensity during the cutting directly changes the size of the prepits or the groove width, this unnecessary variation of light intensity needs to be suppressed.        
Therefore, with the foregoing in mind, it is an object of the present invention to provide a high precision optical disc cutting apparatus capable of forming an optical disc original master by suppressing an unnecessary deflection component and an unnecessary light intensity change component and a method for manufacturing an optical disc.